The present invention relates to electromechanical positioning actuators and, more particularly, to electromechanical positioning actuators operating within a limited range of positions available thereto.
Electromechanical positioning actuators are used for forcing mechanical loads, or at least portions thereof, to be moved to selected positions usually over a limited range of available positions. The mechanical load may be substantial thus requiring substantial force to cause movement thereof but, typically, the rate at which such a load is to be moved from position to position in the available range is very slow. Slow actuation is desired, for instance, in operating loads such as valves, dampers, shutters and the like in fluid flow control systems to avoid unduly rapid changes in downstream fluid conditions. Often, such slow actuation is achieved by having the relatively rapidly moving motion generator in the actuator connected to some gear ratio reduction arrangement as part of the mechanical load driven thereby. In some such actuators, the mechanical load is moved primarily linearly while in others the motion is rotational.
In electromechanical positioning actuators, a limit to the range of available mechanical load positions is typically provided by placing a normally closed limit switch at or near the desired range limit to there be forced into an open condition by the load having been moved to that location. Typically, there will be such a limit switch provided to establish each end of that range of available load positions. As part of the mechanical load, a cam or a lever is typically moved with the remainder of the load across the range of load positions to intersect with one or the other of such limit switches when that cam or lever reaches the corresponding end of the positioning range as the basis for forcing open this limit switch at that end. Such an opening results in terminating directly, or through a supplementary switching arrangement operated by the limit switch, the delivering currently of electrical energy to the motion generator which is typically an electric motor.
Electrical motors, of course, provide a rotational output on the output shaft thereof which can be directly used in connection with a connected gear train in an angular positioning actuator, but which must be used with some sort of a linkage or rack and pinion arrangement to convert such rotational motion of the motor output shaft to a linear motion for a linear positioning actuator. In situations where a direct current power supply and polarity controller are available, a direct current motor is a good choice for the motion generator in such actuators because such motors can provide substantial torques, and because of the direction of rotation of the motor armature is easily chosen by merely choosing the direction of operating current through that armature, i.e. by choosing the polarity of the voltage supplied across that armature.
Because of the desire to have a relatively slow movement of the mechanical load across the range of available positions therefor, the gearing arrangement connected to the output shaft of the direct current motor will have a relatively high gear ratio to provide a sufficient reduction of rotational velocity to the remainder of the mechanical load. Such high gear ratios in a gear train, for instance, also have the advantage of making the gear train essentially self-locking against forces directed thereagainst by the remaining load such as fluid forces acting on the actuator fluid control member interfacing with the controlled fluid. As a result of using such high ratios, there will be no or very little motion of the mechanical load once the limit switch opens, including very little additional motion of the cam or lever in the mechanical load used to force open that limit switch. This is because the angular momentum of the motor armature due to its inertia will be quickly damped before many further armature rotations following the termination of supplying electrical energy thereto so that a high gear ratio will mean very little motion of the gear train output shaft.
Many fast, or snap-acting, limit switches have mechanical arrangement therein allowing a position difference between where the cam of lever forces open the contacts in the switch and where the withdrawal of that cam or lever allows the normally closed switch to reclose by permitting those contacts to again come against one another. However, such limit switches are relatively expensive, leading to a desire to use cheaper, slow-acting limit switches which do not have a significant position difference for the forcing cam or lever in opening and closing the contacts of that switch. Such a situation leaves a substantial possibility of the occurrence of arcing between the contacts of such a limit switch when forced just slightly apart, or even the reclosing of that switch after these contacts are initially forced apart just slightly because of residual compliance in that mechanical arrangement.
Such a situation can result in the establishment of a limit cycle in the positioning of the switch contacts such that repeated openings and closings of the limit switch occurs, i.e. a dithering occurs in connection with the switch contacts. Such a behavior causes undue wear on components in the positioning actuator including the motor, the gear train and the limit switch itself, and often provides an accompanying noisy clatter. Thus, there is desired to have an open position for the contacts in a slow-acting limit switch which has a gap between the switch contacts that exceeds the required minimum dielectric gap and any movement thereof toward reclosing due to system compliance after the initial separating of those contacts in an opening of that switch.